Ultrasound has been used as a therapeutic technique in physical medicine for many years. The technique is effective as an adjunctive therapy for the treatment of pain, soft tissue injury and joint dysfunction, including osteoarthritis, periarthritis, bursitis, tenosynovitis, and a number of musculoskeletal syndromes.
Ultrasonic therapy relies on mechanical vibration of tissues to cause thermal and other effects, using therapeutic frequencies of, for example, 1 MHz or 3 MHz, respectively. The electrical output from the ultrasonic generator is converted into mechanical vibration through a transducer generally made of crystalline material such as lead zirconate titanate or other synthetic or natural crystals. The ultrasonic output may be continuous wave or pulsed depending on the therapeutic indication. The ultrasonic energy is transferred to the tissue of the patient by means of a coupling medium such as ultrasonic gel or water. Output intensities of 0.1-3 watts/cm.sup.2 are typically used in therapeutic applications.
A problem encountered in ultrasound therapy is the difficulty of verifying that the transducer is actually operational during the procedure. Indicators on the ultrasonic generator serve only as an indirect means for informing the operator that the transducer is apparently energized. However, such indicators do not positively demonstrate that the transducer is actually emitting the ultrasound radiation.
The present invention provides a convenient indicator unit which contains a liquid crystal layer. The liquid crystal layer produces a definite color pattern when an active ultrasound transducer is brought in contact with the upper surface of the indicator unit. The unit is constructed so that this color pattern persists for a substantial time interval while the transducer is in use.
Liquid crystals were first discovered about 100 years ago. These crystals are organic compounds derived from cholesterol which exhibit both the flow properties of a liquid and the optical properties of a crystal. Specifically, liquid crystals have the property of scattering light selectively. As liquid crystals are warmed, bright reds, yellows, greens, blues and violet colors appear. Upon cooling, the colors reverse. Liquid crystals became a commercial practicality about 30 years ago when a room temperature mesophase compound was synthesized. This led to the formulation of liquid crystals with a wide variety of temperature ranges.